PLot Moment vs rotation for lumped zero length

[NicolaTarque]

Dear all,
I was analyzing a 2D frame model with lumped zero length at beam and columns. Watching the tutorials I have realized that the rotation can be seen at Material Strain/S23 for each node. The moment can be seen at Material Stress/S23.
I would like to extract the results at each lumped hinge (zero length) moment vs curvature, is it possible? Becasue I would like to know which hinge goes under inelastic deformations and plot the constitutive law.
For example, If I have one story one bay frame, I will have 8 hinges for the 2 columns and 2 hinges for the beam, then I am expevted to plot 8 Moment vs Rotation results.
Thanks,
Nicola

[STKO Team]

Dear Nicola,

First, let me explain how you should interpret the results of a zeroLength element:
A zero length element is a 2-node element with 1 integration point. So every time you want to visualize results on a zeroLength, use the GaussPlot.

The zeroLength element in OpenSees can have up to 6 materials in the 6 DOFs, but it is not mandatory that you set a material in every DOF. For example if you want to release one DOF, don't include it in the defintion of the ZeroLength Material property.

Now the results available for a zeroLength are:

1. material.stress
2. material.strain
3. material.somethingElse

The first 2 are always available, while the third one, depends on the material model you are using in the zeroLength, for example assuming that your material has the output "damage", you can use "materia.damage"

That being said, If you ask for "material.stress" and you set a material only on 2 of the 6 DOFs for the zeroLength, then your output will have 2 components [sigma(1); sigma(2)]. If instead you set 6 DOFs, it will have 6 components, however since the name is "stress" STKO thinks it is a 3D stress tensor in Voigt notation, that's why you see [S11,S22,S33,S12,S23,S13 + all the invariants]. This is unfortunately something out of our control, and we are working on a solution to let STKO understand that material.stress with 6 components is not a stress tensor when computed on a zeroLength.

Now for your second question:
If you want to extract a chart of Moment-Rotation of all zeroLengths, do the following:

1. create a GaussPlot, so that you can select gauss points
2. select all the gauss points of the zeroLength elements you want to extract Moment-Rotation from
3. use the Extract Chart Data command

Let me know if this hepls